1. Field of the Invention
The present invention relates to methods and apparatus for maintenance and improvement of dredge disposal containment areas. In one particular aspect, the invention relates to methods and apparatus for constructing ditching in low ground pressure areas such as containment areas for dewatering and drying of spoil. In another particular aspect, the invention relates to methods and apparatus for removing the dried spoil from the containment area.
2. Description of the Related Art
Dredge disposal or containment areas are located throughout the United States for the disposal of dredge spoil which has been dredged from waterways or spent ore or waste from mining operations. This spoil is classified as hazardous by the EPA since, for example, the sludge dredged from waterways contains oil and other heavy metals which have sunk to the bottom of the waterway. Since this spoil has been classified as hazardous waste and because the land used for a containment area thereafter becomes unusable for any other purpose, no new containment areas are currently being built for the disposal of this spoil. Thus, maximum utilization of existing containment areas is required.
Containment areas may be of various sizes and are generally oval shaped. Generally, a containment area includes a levy which extends around the perimeter of the containment area with the crown of the levy being well above the containment area for the spoil. The levy starts out maybe only a few feet high. Then, as spoil material in the containment area is dried, the dried spoil is removed and added to the top of the levy, raising its height and providing for additional space in the containment area for additional spoil. Typically the top of the levy is flat and about 10-12 feet wide so that a vehicle can drive on the levy. On the inside of the levy there is a berm or work shelf which then slopes down into the pit or into a large ditch which has been dug around the perimeter of the containment area inside the levy. The berm may be 30 to 50 feet wide.
The containment area is typically sloped such that one end is higher than the other end. As the spoil is transported into the containment area, such as through a discharge pipe, the solids drop first into the higher end of the containment area with the fines flowing to the lower end of the containment area. Often the containment area has a two foot drop from one end to the other.
One or more weirs are located in the perimeter ditch to allow the run off of the water draining from the spoil. The weirs typically include four vertical I-beams having horizontal boards extending between the vertical supports. Typically the boards of the weirs are three inches thick and forty-eight inches long. Boards are added to the weir as the level of the containment area is raised through the addition of more spoil thus allowing the water to run over the top boards and into the weir. The weirs are of different sizes depending upon the amount of anticipated water volume. A ditch or run off pipe communicates with the weir to allow the water to flow or be pumped to an adjacent area such as, for example, a river. There may be as many as five pipes carrying water from each weir.
The containment area also includes interior ditches extending across the width of the containment area generally on 50 foot centers. These interior ditches are sloped in the direction of the perimeter ditch to drain the water into the perimeter ditch. Because they cross the pit of the containment area, the interior ditches have proven difficult to construct.
Digging vehicles are used currently to dig the interior ditches and these vehicles include rotary ditchers which dig various width ditches, such as three or six foot wide ditches. The blades of the rotary ditchers have hard steel teeth and rotate at approximately 270 rpms. As the digging vehicle digs an interior ditch, the spoil being removed from the ditch is thrown up to 150 feet on each side of the ditch so as not to obstruct the water drainage into and down the ditch.
The pit of the containment area is difficult for equipment to operate in because the spoil is pumped into the containment area is in the form of a slush or sludge which provides little or no ground support. After being removed from a river or other body of water where it has remained under pressure, the spoil tends to expand and absorb and hold water. Typically, the ground pressure of the spoil in the containment area for supporting the digging vehicle is in the range of 0.3 psi to 1.0 psi. The containment area may have a depth of up to 50 or 60 feet of this slush or sludge. Since the spoil essentially provides no ground pressure to support a vehicle, specially designed digging vehicles must be used for digging the interior ditches. Typically, a digging vehicle weighs approximately 30,000 pounds and includes some type of flotation for moving across the water suspended spoil which provides some limited buoyancy because of the water in the spoil. Digging vehicles used in digging interior ditches across containment areas are manufactured by the Dondi Corporation or the Moletti Corporation, both of Italy, or the John Deere Company. Some of these vehicles use rubber flotation tires while others use continuous tracks. The rubber tired vehicles typically include six to eight foot diameter flotation tires. These tires permit the digging vehicle to remain "afloat" in the spoil, but, unfortunately, they do not provide sufficient traction in wet spoil so that ditches can be dug.
One currently practiced method for digging the interior ditches includes setting up the digging vehicle adjacent one side of the containment area and hooking a cable onto the digging vehicle. The other end of the cable is attached to a bulldozer with the cable extending around a deadman or pulley located on the opposite side of the containment area from the digging vehicle. The cable is not wound onto a spool but is dragged around the containment area behind the bulldozer. The bulldozer then drives along the crown of the levy pulling the digging vehicle across the containment area as it digs an interior ditch. In this method, it is preferred that one long continuous cable be used since the connection for two sections of cable may not pass through the deadman pulley. Thus, it is preferred to have a cable which is at least as long as the distance across the containment area. If the cable breaks, it is necessary to attach the ends of the cable such that the connection will pass through the deadman pulley.
This procedure is very slow and labor intensive. The long cable must be dragged around the containment area by the bulldozer and each time properly aligned with the new interior ditch to be cleaned or dug. Each time a new ditch is to be dug, it is necessary to lay out the cables along the top of the levy and across the containment area.
In pulling the digging vehicle across the containment area to dig an interior ditch, the digging vehicle is pulled with a slow, steady constant movement. The operators of the digging vehicle and bulldozer have radios allowing them to communicate and control the speed of the digging vehicle as it is pulled. The speed of the digging vehicle depends upon the consistency of the spoil in the containment area through which the interior ditch is being dug. If the spoil is very soft, the ditch can be cut faster where if the spoil has been allowed to dry and crust over, it is necessary that the digging vehicle be pulled at a slower speed.
The amount of line pull on the cable which is required to pull one of the digging vehicles across the containment area is determined by the weight of the digging vehicle and the drag created by the digging operation. The digging vehicle sinks into the spoil such that a wall of sludge and slush is pushed in front of the digging vehicle as it is pulled across the containment area. Often, the amount of line pull on the cable required to pull the digging vehicle across the containment area is in the range of 40,000 to 60,000 pounds.
Attempts have been made in the past to use hydraulic winches to pull the digging vehicle across the containment area. These attempts have largely been unsuccessful. A hydraulic winch is set at a torque level based on release valves, the amount of hydraulic flow (gpm) and the psi of the pump. The hydraulic winch also is regulated by a series of slip connections floating in oil. These combined create the line pull tonnage of the hydraulic winch. The hydraulic motor is connected directly to the shaft of the spool for the cable. Once the hydraulic winch reaches a certain amount of torque, there is a bypass. However, the torque designed for the hydraulic winch can easily be exceeded in pulling the digging vehicle such that the bypass may not be affected.
Hydraulic winches are rated by the tons of line pull on the first wrap of the cable around the spool. The first wrap of the cable might have 5 tons of line pull but every time a wrap of cable is added to the spool, the amount of line pull decreases. It has been observed that a 1/2 inch cable wrapped on the spool of a hydraulic winch was not strong enough to pull a ditching vehicle across the containment area and would often break. Thus, it was necessary to use a 9/16 inch cable. The spool of the hydraulic winch would only accommodate approximately 800 feet of 9/16 inch diameter cable. However, each additional wrap on the spool reduced the line pull of the hydraulic winch. One such hydraulic winch was used by ARDCO, now Pettibone, of Houston, Tex., (713)433-6751 and was mounted on a rubber tired vehicle.
Because hydraulic winches lose torque as more wraps of cable are added to the spool, it was necessary to use multiple sections of cable limited to what the winch could carry (typically on the order of 500 feet) so that only one cable section would be wrapped onto the spool of the winch at one time to ensure that there was sufficient torque to pull the digging vehicle across the containment area. Using this method, the winch spool only accommodates a few hundred feet of cable and then the cable has to be unhooked from the winch, respooled onto a separate spool, and then a new section of cable rehooked again to the winch spool to pull that cable section onto the spool. This procedure is cumbersome, requires a separate spool for the cable, and oftentimes does not have sufficient torque to pull the digging vehicle across the containment area.
For further information on spoil disposal and crust management of spoil, see the attached document entitled "Wilco Marsh Buggies, Inc.: Experts in De-Watering and Disposal Area Construction" incorporated herein by reference.
In some large containment areas, the width of the containment area pit may be two miles. It can be appreciated that a hydraulic winch has difficulty in accommodating 12,000 feet of cable to handle this large distance. There is no known hydraulic winch which is capable of providing 60,000 lbs. of line pull using 12,000 feet of cable.